Process for producing metal borides on the surface of metals



April 28, 1970 M. BASCHE 3, 77

PROCESS FOR PRODUCING METAL BORIDES ON THE SURFACE OF METALS Filed Jan. 11. 1967 4g" f u9 2 INVENTOR MALCOLM BASCHE United States Patent 3,508,977 PROCESS FOR PRODUCING METAL BORIDES ON THE SURFACE OF METALS Malcolm Basche, West Hartford, Conn., assignor to Union Carbide Corporation, a corporation of New York Filed Jan. 11, 1967, Ser. No. 608,644 Int. Cl. C23c 11/08, 9/10; C23f 7/00 U.S. Cl. 1486.11 7 Claims ABSTRACT OF THE DISCLOSURE A process for coating a metal object with a metallic boride which consists of submerging a heated metal object into a liquid bath of boron trichloride. Alternatively, the metal object can be submerged in the bath of boron trichloride and thereafter heated to a temperature sufficient to cause a boride coating to occur on the surface of the metal object.

boride composites can be used to form many products such as laboratory ware, crucibles, and the like.

Several processes have been advanced for forming a metal boride coating on metal substrates. One such process comprises placing the metal object to be coated in a coating pack containing a source of boron and a halide and heating until boron diffuses into the surface of the metal. Such a process is tedious and time consuming since each metal object that is to be coated must be separately placed in the coating pack and the process carried out to completion before the next object can be processed. Furthermore, it is extremely difiicult using this and other prior art processes to form a metal boride coating on intricately shaped metal substrates. Therefore, it would be most desirable in terms of cost and efficiency, if in place of the prior art batch processes a continuous coating process could be provided.

It is a primary object of this invention, therefore, to provide such a process whereby metal objects or substrates are continuously coated with metal borides.

It is another primary object of this invention to provide a process for coating intricately shaped metal objects with metal borides.

It is another object of this invention to provide a process for coating metal substrates with metal borides Wherein the processing time is reduced to a minimum.

These and other objects of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a cross-sectional view apparatus which may be suitably employed in the process of the invention; and

FIGURE 2 is a cross-sectional view of another embodiment of an apparatus which is suitably employed in the process of the invention.

Broadly stated, the objects of the invention are accomplished by placing the metal object to be coated in a pool of liquid boron trichloride and heating the object until a metal boride coating results. The reaction which results, metal +BCl metal boride+Cl takes place at the surface of the metal and a hard boride coating forms.

Any metal which will readily combine with boron to form the metal boride is suitable as the substrate material. Such metals include tantalum, molybdenum, zirconium, niobium, titanium, and tungsten.

In the process of the invention, the boron trichloride is maintained in a liquid state. The liquid boron trichloride may be provided by introducing this compound in the gaseous state and cooling it to below 12 C. Boron trichloride itself can be made by passing chlorine over heated boron or various alloys of boron. In an alternative process, it may be prepared by the action of phosphorous pentachloride or oxychloride or boric acid.

Referring now to the drawing, there is illustrated in FIGURE 1 an apparatus 10 which is suitably employed in the process of the invention. The apparatus 10 comprises a container 12, an inlet valve 14 and an exhaust valve 16. The metal object to be coated 18 is submerged beneath liquid boron trichloride which is designated by the numeral 20. Heating means such as induction coils 22 are also provided. An outer container 24 confines a fluid such as water 26 which is cooled to a sufficiently low temperature to maintain the boron trichloride in a liquid state.

In operation, boron trichloride enters the container 12 in a gaseous state through the inlet valve 14. Due to the surrounding low temperature which is provided by the cooled fluid 26 the boron trichloride liquefies and forms a pool at the bottom of the container 12. The metal object to be coated 18 is then introduced into the chamber 12 through an opening which is sealed by the plug 15, and is submerged in the liquefied boron trichloride. The metal object is then heated to a tempearture sufficient to cause a boride coating to occur on the surface of the object. This temperature depends on the particular metal to be treated and for refractory metals is usually between about 800 C. and about 1500 C. During the process, chlorine gas is released from the liquid and passes out of the outlet valve 16. In an alternative procedure, the object may be first heated to the desired temperature and then plunged into the liquid and maintained there until the coating forms.

It will be appreciated that objects of any shape can be submerged in the liquid and an excellent boride coating formed either on the outer surface or on the inside surface of the object. If the object is completely submerged in the liquid, the pressure is substantially equal on all surfaces and as a result, a uniform coating is formed on each surface. In addition, an object of any shape will be completely covered with the liquid at its surfaces and a boride coating will form and adhere to a surface of any configuration.

The apparatus of FIGURE 1 may readily be modified so as to provide an apparatus which is adaptable to continuous process. Such an apparatus is illustrated in FIG- URE 2.

Referring now to FIGURE 2, there is shown a modified apparatus for use in the process of the invention. This apparatus 28 comprises an inner container 30 and an outer container 32. The inner container houses liquefied boron trichloride 34 which is kept in a liquid state by the presence of the ice water designated by the numeral 36. Metal, such as wire 38 is positioned over the roller 40, through the gas tight opening 42 in the side of the chamber 30, over rollers 44 and 46 through the liquid boron trichloride 34, over rollers 48 and 50 and out of the chamber at the gas tight opening 52. The wire is collected on roller 54. While the wire is present in the liquefied boron trichloride it is heated to a suitable temperature by the heating means 56. During operation, the wire is passed over the rollers and through the liquid chloride by motor means (not shown). In this manner the metal wire 38 is continuously coated without interruption of the process or disruption or disassembly of the apparatus.

An an example of the invention the apparatus illustrated in the drawing was employed to process and coat a tungsten wire having a diameter of .010". Boron trichloride gas was introduced into a previously evacuated and purged container at atmospheric pressure and was liquefied by surrounding the container with ice water. The tungsten wire was introduced into the liquefied boron trichloride and was completely covered therewith. The wire was then heated to a temperature of approximately 1200 C. and was completely converted to boride in 2%. minutes. Non-liquefied gas and the reaction by-product, chlorine gas, were removed from the container through an outlet valve. The resultant product had no tungsten core visible and no coating since the entire wire was converted to boride. The wire had a micro-hardness of approximately 2500 Knoop.

While the above example was carried out with the tungsten wire, the same process could obviously be employed for such other metals as tantalum, molybdenum, zirconium, niobium, titanium and the like. In addition, the surface to be coated could first be coated with an appropriate metal and then immersed in the bath and converted to the boride. Heating could be carried out by induction means or internal resistance heating wherever applicable.

It is obvious that the metal object need not be completely submerged in the liquid boron trichloride, but only those surfaces of the object which are to be coated are required to be covered by the liquid. Thus, if it is desirable tocoat only one surface of the object, the process i readily employed to provide such a coating.

From the foregoing it will be appreciated that the process of the invention provides a means whereby even the most intricately shaped bodies can be coated with a metal boride coating in a relatively short period of time. In addition, the process of the invention, unlike the prior art batch processes wherein the metal to be coated is packed in a granular composite, provides a continuous means of coating metal objects. Furthermore, metal objects can be individually coated in an extremely rapid and etficient manner by first heating the object to a suitable temperature and then plunging it into the liquid trichloride until a coating occurs. In this manner, a great number of objects can be quickly processed by being preheated and then submerged in the liquid trichloride either individually or in groups.

By the term metal object as used herein is meant an object of any shape and composed of any metal which exhibits an afiinity for combining with boron to form the m tal boride. The object may be of any size subject to the obvious limitation which is apparent to one skilled in the art that the volume of the liquid trichloride must be greater than the volume of the portion of the metal object submerged therein. Since the liquid is converted to the gaseous state in the area immediately surrounding the submerged metal object when the metal object is raised to a high temperature, a ratio of liquid volume to the volume of the portion of metal object submerged therein of at least 2:1 is required.

What is claimed is:

1. A process for diitusion coating of boron into the surface of metal objects comprising (a) providing a liquid pool of boron trichloride, (b) submerging the metal surface to be coated into said pool, (c) heating said metal object to a temperature sufficient to cause a boride coating to occur at the surface of said object, and (d) removing said object from said pool.

2. The process of claim 1 wherein said metal object is composed of a material selected from the group comprising tantalum, molybdenum, zirconium, niobium, titanium and tungsten.

3. The process of claim 2 wh rein said temperature is between 800 C. and 1500 C.

4. A process for the production of a boride coating on the surface of a suitable metal object comprising (a) forming a liquid pool of boron trichloride, (b) passing the metal surface to be coated through said pool, and (c) heating said metal object while said surface is passing through said liquid pool to a temperature sufficient to cause a boride coating to form at said surface.

5. The process of claim 4wherein said metal object is composed of a material selected from the group comprising tantalum, molybdenum, zirconium, niobium, titanium and tungsten.

6. The process of claim 5 wherein said temperature is between 800 C. and 1500 C.

7. A process for the production of a boride coating on the surface of a metal object selected from the group comprising tantalum, molybdenum, zirconium, niobium, titanium and tungsten, said process comprising:

(a) providing a liquid pool of boron trichloride,

(b) h ating said metal object to a temperature of between 800 C. and 1500 C., and thereafter (0) plunging said surface to be coated into said liquid boron trichloride,

(d) maintaining said surface in said boron trichloride until said coating forms thereon, and

(e) removing said metal object from said boron trichloride.

References Cited UNITED STATES PATENTS 455,187 6/1891 Erlwein. 1,987,576 1/1935 Moers 117231 2,523,461 9/1950 Young et a1 117-113 X 2,852,409 9/1958 Roe 1l7--21 3,029,162 4/1962 Samuel et al 1l7107 3,201,286 8/1965 Hill et al 1486.l1

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

